As a radio access scheme of a PHS system or the like, a TDMA (Time Division Multiple Access)/TDD (Time Division Duplex) scheme has been employed in which a TDMA system and a TDD system are combined. Recently, an OFDMA (Orthogonal Frequency Division Multiplexing Access) system employing an OFDMA scheme based on an OFDM (Orthogonal Frequency Division Multiplexing) technique has been proposed.
The OFDM is a scheme of dividing a carrier for data modulation into a plurality of “subcarriers” (subdivided carriers) orthogonal to each other and distributing and transmitting a data signal in each subcarrier.
Hereinafter, an overview of the OFDM scheme will be described.
FIG. 15 is a block diagram illustrating a configuration of the OFDM modulation device used at a transmission side. Transmission data is input to the OFDM modulation device. The transmission data is supplied to a serial/parallel conversion unit 201 and converted into data including a plurality of low-speed transmission symbols. That is, a plurality of low-speed digital signals are generated by dividing transmission information. This parallel data is supplied to an inverse fast Fourier transformation (IFFT) unit 202.
The parallel data is allocated to each subcarrier configuring OFDM and mapped in a frequency domain. Here, the subcarrier is modulated by BPSK, QPSK, 16QAM, 64QAM, or the like. The mapping data is converted from frequency-domain transmission data to time-domain transmission data by performing an IFFT operation. Accordingly, multi-carrier modulation signals into which a plurality of subcarriers orthogonal to each other are modulated independently are generated. An output of the IFFT unit 202 is supplied to a guard interval addition unit 203.
As shown in FIG. 16, the guard interval addition unit 203 sets a rear part of an effective symbol of transmission data as a guard interval and adds its copy to a front part of an effective symbol period for each transmission symbol. A base-band signal obtained by the guard interval addition unit is supplied to an orthogonal modulation unit 204.
The orthogonal modulation unit 204 orthogonally modulates a base-band OFDM signal supplied from the guard interval addition unit 203 using a carrier signal supplied from a local oscillator 205 of the OFDM modulation device, and performs frequency conversion into intermediate frequency (IF) signal or a radio frequency (RF) signal. That is, after frequency-converting the base-band signal into a desired transmission frequency band, the orthogonal modulation unit outputs it to a transmission path.
FIG. 17 is a block diagram illustrating a configuration of an OFDM demodulation device to be used in a receiving side. An OFDM signal generated by the OFDM modulation device of FIG. 15 is input to the OFDM demodulation device through a predetermined transmission path.
An OFDM reception signal input to the OFDM demodulation device is supplied to the orthogonal demodulation unit 211. The orthogonal demodulation unit 211 orthogonally demodulates the OFDM reception signal using a carrier signal supplied from a local oscillator 212 of the OFDM demodulation device, performs frequency conversion from a RF signal or an IF signal into a base-band signal, and obtain a base-band OFDM signal. The OFDM signal is supplied to a guard interval removing unit 213.
The guard interval removing unit 213 removes a signal added by the guard interval addition unit 203 of the OFDM modulation device according to a timing signal supplied from a symbol timing synchronization unit (not shown). A signal obtained by the guard interval removing unit 203 is supplied to a fast Fourier transformation (FFT) unit 214.
The FFT unit 214 performs conversion to frequency-domain reception data by performing an FFT operation on input time-domain reception data. De-mapping is performed in the frequency domain and parallel data is generated for each subcarrier. Here, the demodulation to the modulation of BPSK, QPSK, 16QAM, 64QAM, or the like performed for each subcarrier is performed. Parallel data obtained by the FFT unit 214 is supplied to a parallel/serial conversion unit 215 and output as reception data.
The OFDM is a scheme for dividing a carrier into a plurality of subcarriers. The OFDMA is a scheme for collecting and grouping a plurality of subcarriers among the subcarriers in the OFDM and performing multiplex communication by allocating one or more groups to each user. Each group is called a subchannel. That is, each user performs a communication using one or more allocated subchannels. According to a communication data amount, a propagation environment, and the like, subchannels are adaptively increased and decreased, and allocated.
In a PHS system employing such OFDMA scheme, a frame is configured so as to include, for example, four time slots, where the vertical axis represents a frequency and the horizontal axis represents a time. A downlink period and an uplink period are both divided into a plurality of frequency bands in the frequency axis. The subchannel allocated to the first frequency band is called control subchannel and is used as a control channel (CCH). The other frequency bands are configured as traffic channels (TCH) each including a plurality of subchannels.
When a mobile station moves to a boundary of a cell which is a range wirelessly communicable with a base station or when radio waves from the base station in communication are weakened to disable the communication, switching to a base station having strong radio waves in another cell is performed after or before the radio waves are weakened, which is called a handover.
Such a handover is performed as follows, for example, as described in Patent Document 1.
FIG. 18 shows a situation in which a wireless communicable range with a base station BTS 101 is a cell 112 and a wireless communicable range with a base station BTS 102 is a cell 113, a mobile station MS 107 is moving from the cell 112 of the base station BTS 101 as a handover source to the cell 113 of the base station BTS 102 as a handover destination.
The mobile station MS 107 monitors the power of the mobile station MS 107 during communication with the base station BTS 101 and notifies a center MSC 111 of the monitoring result when a handover to the base station BTS 102 is predicted. The mobile station MS 107 receives a list of neighbor base stations, monitors signals from the neighbor base stations at a constant time interval on the basis of the list, and notifies the base station BTS 101 of the monitoring result. When a boundary condition of the handover is satisfied, a message is transmitted to a base station controller BSC 109. The message includes a parameter for recognizing the mobile station MS and data on a new channel (time slot) to be used in a communication between the mobile station MS and the base station BTS 102 from now on. The handover to the base station BTS 102 is started under the control of the MSC 111 when the mobile station MS 107 is in the overlap range of the cell 112 and the cell 113, and the handover is ended when the mobile station 107 enters the cell 113.
Patent Document 1: JP-A-2002-300628 (pages 3 and 4 and FIG. 10)